The invention relates to a cathode ray tube comprising
a display screen for converting an electron-optical image into a light image, and
an electron-optical system comprising
electron sources, juxtaposed in a plane, for emitting electrons;
a beam-shaping section for forming a first outer electron beam, a middle electron beam and a second outer electron beam from the electrons emitted by the respective electron sources;
a main lens for focusing the electron beams on the display screen;
deflection means for deflecting the electron beams across the display screen, and
a DAF section for dynamically adapting the focusing and astigmatism of the electron beams in dependence upon a landing spot of the electron beams on the display screen.
The invention also relates to a picture display device comprising such a cathode ray tube.
An embodiment of such a cathode ray tube is known from U.S. Pat. No. 4,814,670.
In a picture display device comprising the cathode ray tube, three electron beams are generated by the electron gun, which beams are imaged on the display screen. The display screen has lines or dots of phosphors which luminesce when they are impinged upon by one of the electron beams.
For displaying color images, use is made of an electron gun in which three electron beams are generated which are juxtaposed in what is called an xe2x80x9cin-linexe2x80x9d plane. The three electron beams are focused on the display screen by the main lens. The display screen is provided with red, green and blue phosphors. Furthermore, the cathode ray tube is provided with means which ensure that each electron beam lands on its own phosphor, which means comprise, for example, a shadow mask. Each electron beam thus corresponds to one of the colors red, green and blue.
In a frequently used configuration of the electron beams, the first outer electron beam particularly corresponds to the color red, the middle electron beam corresponds to the color green and the second outer electron beam corresponds to the color blue.
The cathode ray tube has deflection means for deflecting the electron beams. Generally, the cathode ray tube has a neck around which magnetic deflection means are arranged. For deflecting the electron beams, the deflection means receive, in operation, a deflection current which is synchronized with a picture signal received by the picture display device.
As the electron beams are deflected by the deflection means, the electrons cover a longer path between the electron source and the landing spot on the display screen. More particularly, the electrons cover a longer path between the main lens and the display screen, in dependence upon the extent of deflection. As a result, the electron beam is out of focus on at least a part of the display screen and is imaged as a relatively hazy picture.
Furthermore, when deflecting the electrons, the deflection means act as an electron-optical quadrupolar lens which will hereinafter also be referred to as deflection lens. Due to this quadrupolar lens, astigmatism occurs and the shape of the electron beam changes in dependence upon the deflection. The strength of the quadrupolar lens increases with an increasing extent of deflection of the electron beam.
The resolution of a cathode ray tube is dependent on the size and shape of the image of the electron beam, referred to as the spot. The change of the extent of focusing and of the astigmatism of the electron beam due to the deflection reduces the quality of a spot. Consequently, the resolution of the cathode ray tube decreases, particularly in the corners of the display screen.
To reduce this effect, the electron gun is provided with a DAF section as is known from the above-mentioned U.S. Pat. No. 4,814,670. Particularly, the DAF section comprises an intermediate electrode which is provided with horizontal, elongated apertures on the side facing the focusing electrode. The focusing electrode is provided with vertical elongated apertures. xe2x80x9cHorizontalxe2x80x9d is herein understood to mean the direction parallel to the xe2x80x9cin-linexe2x80x9d plane and perpendicular to the direction of propagation of the electrons. xe2x80x9cVerticalxe2x80x9d is herein understood to mean the direction perpendicular to the xe2x80x9cin-linexe2x80x9d plane.
In operation, a dynamic focusing voltage is applied to the intermediate electrode so that an electron-optical quadrupolar lens is formed between the focusing electrode and the intermediate electrode. The strength of the main lens can also be adapted by means of the dynamic focusing voltage.
Generally, the deflection means are self-convergent in the horizontal direction. This means that the electron beams in the horizontal direction are substantially in focus throughout the display screen, which is at the expense of an increased overfocusing in the vertical direction.
The known color electron gun has a substantially equal design for the three electron beams. Consequently, the DAF section for the three electron beams has the same effect, i.e. the electron-optical quadrupolar lens formed, in operation, between the focusing electrode and the intermediate electrode has an equal strength for all of the three electron beams.
However, since the three electron beams are juxtaposed in the in-line plane and are situated at a given mutual distance from each other, for example, at a distance of 6 mm at the location of the deflection means, they travel along different paths through the magnetic field of the deflection means deflecting the three electron beams. Consequently, the deflection lens has mutually different strengths for the three electron beams. This effect is referred to as xe2x80x9ccolor-dependent defocusingxe2x80x9d.
It has been found that color-dependent defocusing affects the resolution of the cathode ray tube to a considerable extent, notably in a cathode ray tube for a computer monitor, a cathode ray tube having a relatively large deflection angle of the electron beams and a cathode ray tube without a shadow mask, referred to as Flat Intelligent Tracking (FIT) cathode ray tube.
Generally, the DAF section is adjusted in such a way that, in operation, the middle electron beam is substantially in focus throughout the display screen, while the outer electron beams are then no longer in focus at the edges and particularly in the corners of the display screen.
The color-dependent defocusing is then notably visible because the first outer electron beam is overfocused in the vertical direction on the east side of the display screen, which, viewed from the exterior, is the right-hand side. Generally, the first outer electron beam corresponds to the color red, and in this case the red spot is hazy. On a relatively large color monitor with a relatively high resolution, red characters may be out of focus on the east side of the screen.
Moreover, the second outer electron beam is overfocused in the vertical direction on the west side of the screen, which, viewed from the exterior, is the left-hand side. Generally, the second outer electron beam corresponds to the color blue and in this case the blue spot is hazy. On a relatively large color monitor with a relatively high resolution, blue characters may be out of focus on the west side of the screen.
The known cathode ray tube has the drawback that color-dependent defocusing occurs.
It is an object of the invention to provide a cathode ray tube having an improved focusing of the outer and inner electron beams at the edges, and particularly in the corners, of the display screen.
It is a further object of the invention to provide a picture display device comprising such a cathode ray tube.
In the cathode ray tube according to the invention, the first object is achieved in that the DAF section comprises a first electron lens, which has mutually different strengths for the electron beams, and a second electron lens, which has mutually different strengths for the electron beams, the strengths of the second electron lens being changeable independently of the strengths of the first electron lens.
In operation, the DAF section in the cathode ray tube according to the invention compensates the color-dependent defocusing because there is always a linear combination of the first and the second electron lens that acts on the electron beams.
A cathode ray tube in which the color-dependent defocusing is partly inhibited by placing an extra electrode between the focusing electrode and the DAF section is known from patent application EP-A-0 899 768. By means of this electrode, an electron-optical quadrupolar lens can be formed, in operation, for the outer electron beams. This lens acts on the outer electron beams with the same strength but with opposite sign.
However, it has been found that color-dependent defocusing is asymmetrical, a color-dependent defocusing error xcex94 substantially having a first and fifth-order dependence upon the landing spot of the electron beam on the screen. For example, along the line axis, the color-dependent defocusing error is dependent on the distance X between the landing spot and the field axis, in accordance with
xcex94=c1X+c5X5xe2x80x83xe2x80x83(1) 
in which c1 and c5 are constants. The partial solution described in EP-A-0 899 768 may sufficiently compensate the linear term, however, the fifth-order term is dominant in cathode ray tubes having relatively large deflection angles of the electron beams, i.e. relatively large values of X.
In the cathode ray tube according to the invention, color-dependent defocusing can be compensated asymmetrically so that an improved focusing of the outer electron beams at the edges, and particularly in the corners, of the display screen can be achieved.
Generally, the first and the second electron lens are astigmatic, i.e. the first and the second electron lens focus in a first direction and defocus in a second direction which is perpendicular to the first direction. For example, both electron lenses focus in a horizontal direction and both defocus in a vertical direction.
The cathode ray tube according to the invention provides a further advantage if the deflection means are self-convergent in a first direction, for example, the horizontal direction. In this case, the strength of the main lens should be adaptable in such a way that it compensates the effect of the first electron lens and of the second electron lens in the first direction. Consequently, in the first direction, the electron beams can be held substantially in focus throughout the display screen.
The partial solution which is known from the above-mentioned patent application EP-A-0 899 768 consists of an extra electron-optical quadrupolar lens which is placed between the focusing electrode and the DAF section and acts on the outer electron beams. The effect of this extra lens cannot be compensated by adapting the main lens so that this solution changes the focusing of the outer electron beams in the first direction.
In an embodiment of the cathode ray tube, the strength of the first outer electron beam is larger, in operation, for the first electron lens than the strength of the middle electron beam, and the strength of the second outer electron beam is larger than the strength of the middle electron beam. In a formula, this can be indicated by S1R greater than S1G greater than S1B, wherein the strength of the first electron lens for the first outer electron beam is denoted by S1R, for the middle electron beam by S1G and for the second outer electron beam by S1B.
Alternatively, the strength of the first outer electron beam may be smaller than the strength of the middle electron beam for the first electron lens, and the strength of the second outer electron beam may be larger than the strength of the middle electron beam. In a formula, this can be indicated by S1R less than S1G less than S1B.
In a further advantageous embodiment, the strengths of the first and the second electron lens are equal for the middle electron beam, the strength of the first electron lens for the first outer electron beam is equal to the strength of the second electron lens for the second outer electron beam, and the strength of the first electron lens for the second outer electron beam is equal to the strength of the second electron lens for the first outer electron beam. In a formula, this can be indicated by S1G=S2G, S1R=S2B and S1B=S2R, in which the strength of the second electron lens for the first outer electron beam is indicated by S2R, for the middle electron beam by S2G and for the second outer electron beam by S2B.
This is advantageous because the first outer electron beam generally has a substantially equal behavior on the east side of the display screen as the second outer electron beam has on the west side of the display screen, and vice versa.
Generally, it is simplest to realize the electron lenses with electric means. To this end, a focusing electrode is present between the beam-shaping section and the DAF section in a special embodiment of the cathode ray tube, which focusing electrode is provided on the side of the DAF section with apertures, which mutually differ in shape, for passing the electron beams, and the DAF section comprises a first intermediate electrode and a second intermediate electrode, the second intermediate electrode being placed between the first intermediate electrode and the main lens. On the side of the focusing electrode as well as on the side of the second intermediate electrode, the first intermediate electrode is provided with apertures of mutually different shapes for passing the electron beams. On the side of the first intermediate electrode, the second intermediate electrode also has apertures of mutually different shapes for passing the electron beams. In this way, electron lenses with mutually different strengths for the three electron beams can be easily realized by applying an electric field between facing apertures.
Particularly, the first electron lens can be formed, in operation, by applying an electric field between the apertures in the focusing electrode and the apertures facing it in the first intermediate electrode, the first intermediate electrode receiving a dynamic voltage Vdyn1. In operation, the second electron lens can be formed by applying a second electric field between the apertures in the second intermediate electrode and the apertures facing it in the first intermediate electrode, the second intermediate electrode receiving a second dynamic voltage Vdyn2. The strengths of the second electron lens are proportional to a difference voltage Vdyn2xe2x88x92Vdyn1. The strength of the main lens is also adapted by the second dynamic voltage Vdyn2.
Generally, both the first dynamic voltage Vdyn1 and the second dynamic voltage Vdyn2 are synchronized with a picture signal received by the cathode ray tube. However, their amplitudes are independent of each other so that the strengths of the second electron lens are changeable independently of the strengths of the first electron lens.
Furthermore, the DAF section may comprise a third electron lens which has mutually different strengths for the electron beams, the strengths of the third electron lens being changeable independently of the strengths of the first and the second electron lens. The focusing for each color can be adapted independently with such a DAF section. This provides an advantage if the color-dependent defocusing has a very strong asymmetry. Moreover, this may be advantageous if strict requirements are imposed on the spot size, such as in a FIT tube, in which the spot size in a direction perpendicular to the phosphor tracks, which is generally the vertical direction, should be limited to about 300 micrometers so as to prevent color errors.
An extra electrode configuration which precedes the DAF section and with which the focusing can be adapted independently for each electron beam, is known per se from patent application JP-A-2000011916. However, in this patent application, use is made of unconventional elements which are expensive and difficult to manufacture in comparison with the traditional electron lenses used in the cathode ray tube according to the invention.
An embodiment of the cathode ray tube, in which a third electron lens is present in the DAF section, has a third intermediate electrode which is placed between the second intermediate electrode and the main lens and is provided on the side of the second intermediate electrode with apertures of mutually different shapes for passing the electron beams. In operation, the third electron lens can then be formed by applying a third electric field between the apertures in the third intermediate electrode and the apertures facing it for passing the electron beams in the second intermediate electrode.
For applying the third electric field, the third intermediate electrode receives a third dynamic voltage Vdyn3. The strengths of the third electron lens are proportional to a difference voltage Vdyn3xe2x88x92Vdyn2. Also the third dynamic voltage Vdyn3 is synchronized with the picture signal but has an amplitude which is independent of Vdynl and Vdyn2. Consequently, the strengths of the third electron lens are changeable independently of the strengths of the first and the second electron lens.
In such an embodiment, the first, second and third intermediate electrodes can be placed in the DAF section in any arbitrary sequence. It will be evident that, in operation, any combination constitutes a first, a second and a third electron lens having strengths which are mutually different for the three electron beams and are changeable independently of each other.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.